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The Vector Field DOP creates a Vector Field data that can be attached to simulation objects and manipulated by solvers. A Vector Field is an axis-aligned box divided into individual voxels. Each voxel is giving a 3d vector.
The meaning of these numbers could vary. They could represent force directions or a color field. The Vector Field is initialized with data from a SOP network.
Note
The field data is imported from SOPs prior to any transformation defined by the Position Data Path.
Parameters
Use SOP Dimensions
Overrides the Divisions, Center, and Size options according the properties of the volume primitive in the specified SOP. This attempts to bring the SOP back at the same size and resolution it was supported.
Note
This may not work if the SOP has been transformed, especially if rotated, as the DOP scalar field has to be axis aligned. Only the first specified volume primitive will be used to calculate the override - the other two axes will be sampled in that space.
SOP Resize Only
Restricts Use SOP Dimenstions option to only override Center and Size. This is useful for customizing the resolution of the vector field.
Two Dimensional
One of the divisions of the voxel grid will be forced to one to create a two dimensional field.
Plane
If set to two dimensional, this plane determines which axes remain unaffected.
Division Method
If non square, the specified size is divided into the given number of divisions of voxels. The sides of these voxels may not be equal, however, possibly leading to distorted simulations.
When an axis is specified, that axis is considered authoritative for determining the number of divisions. The chosen axis' size will be divided by the uniform divisions to yield the voxel size. The divisions for the other axes will then be adjusted to the closest integer multiple that fits in the required size.
Finally, the size along non-chosen axes will be changed to represent uniform voxel sizes. If the Max Axis option is chosen, the maximum sized axis is used.
When By Size is specified, the Division Size will be used to compute the number of voxels that fit in the given sized box.
Uniform Divisions
The resolution of the key axis on the voxel grid. This allows one to control the overall resolution with one parameter and still preserve uniform voxels. The Uniform Voxels option specifies which axis should be used as the reference - it is usually safest to use the maximum axis.
Divisions
The resolution of the voxel grid that will be used to calculate the smoke object. Higher resolutions allow for finer detail in both the appearance and in the resulting motion. However, doubling the divisions requires eight times the memory.
Since the substepping should be proportional to the voxel size, doubling the divisions may require double the substepping, resulting in sixteen times the simulation time.
Division Size
The explicit size of the voxels. The number of voxels will be computed by fitting an integer number of voxels of this size into the given bounds.
Size
The size of the voxel grid. The size of each voxel will be this divided by the divisions.
Center
The position in world space of the center of the voxel grid.
Voxel Sampling
The overall box will be divided into the specified number of small boxes, or voxels. However, the question remains where on these voxels the field’s vectors should be stored.
Center
The vector will be stored at the center of each voxel. The total number of samples thus matches the number of boxes.
Faces
The vector will be split and each component will be stored on one of the voxel’s faces.
The x component on the x face, the y component on the y face, and the z component on the z face. Along that dimension, the number of samples will be one higher to store the start and end voxel’s face values.
Edges
The vector will be split and each component will be stored on the center of one of the voxel’s edges.
The dimensions involved will be one larger to account for the boundaries.
Corner
The value will be stored on the corners of each voxel.
The number of samples will be one higher in each dimension than the division count to contain all the boundary values.
Closed Ends
Determines the behavior of boundary values.
If set, the boundary will be enforced with the External Direction. Otherwise, the boundary is left free.
X, Y, Z
When closed ends is set, these select which sides will be closed.
Border Type
The behavior when the field is sampled outside of its defined box.
Constant
The external direction will be returned.
Repeat
The field will wrap, returning values from the opposite side of the field.
Streak
The value at the edge of the field closest to the sample will be returned.
Extrapolated
The value at the edge of the field closest to the sample
will be used. However, it will then be added to the
dot product of the voxel size with the difference of voxel
indices. This results in the correct extrapolation of
axis aligned rest fields. For non-axis aligned rest
fields, the 
Gas Rest DOP will set up the ratios properly.
External Direction
What value to enforce on the boundary of the vector field.
Scale Values
The incoming SOP field will be multiplied by this value.
Tolerance
The absolute tolerance to use for lossy compression of the vector field. This can reduce memory usage by detecting constant areas or using a lower bit-depth representation.
A value of 0 only allows lossless compression.
Use 16bit Float
The tiles will be stored using a 16 bit float rather than 32 bit float. This uses half the memory, but at the cost of reduced precision and increased computation.
Note
All computations are still done in 32 bit floats.
Sop Path
The SOP to extract the three volume primitives from.
After the primitives are extracted, this field is erased. To reload the vector data every frame, use Set Always.
Primitive Numbers
Which primitive in the SOP are the volume primitives to correspond to the x, y, and z values for the vector field.
Time
The time to evaluate the SOP at, in seconds. $T is a suitable
value to evaluate at the current global time.
Use Object Transform
Turn on this option to embed the transform from the parent object of the SOP along with the geometry.
Note
This does not change the contents of the vector field.
Re-evaluate SOPs to Interpolate Geometry
Normally when a solver asks for geometry data in a sub-step, the simulation will simply linearly interpolate position data from integral frames. However, this is not exact. Turning this option on re-evaluates the geometry network for each substep. This is more accurate, but can be very expensive.
Slice
Which slice to use. Should be a number between 0 and the number of slices - 1.
Slice Divisions
Number of pieces to cut the volume into along each axis. The total number of pieces, or slices, created will be the product of these numbers. Ie, 2, 3, 4 will create 24 slices.
Overlap Voxels Negative, Positive
Adds a padding on the lower/upper side of the slices. The slices start by dividing space evenly, but then this overlap will cause them to overlap with their neighbors. The field exchange nodes use this overlap to determine what is communicated.
Position Data Path
The optional relative path for Position data. This will be used to
transform the fluid box, allowing for non-axis aligned fluid sims. A value
of ../Position will allow you to attach a 
Position DOP to your fluid object and thus reorient the fluid.
Inputs
First Input
This optional input can be used to control which simulation objects are modified by this node. Any objects connected through this input and which match the Group parameter field will be modified.
If this input is not connected, this node can be used in conjunction with an Apply Data node, or can be used as an input to another data node.
All Other Inputs
If this node has more input connectors, other data nodes can be attached to act as modifiers for the data created by this node.
The specific types of subdata that are meaningful vary from node to
node. Click 
an input connector to see a list of available
data nodes that can be meaningfully attached.
Outputs
First Output
The operation of this output depends on what inputs are connected to this node. If an object stream is input to this node, the output is also an object stream containing the same objects as the input (but with the data from this node attached).
If no object stream is
connected to this node, the output is a data output. This data
output can be connected to an 
Apply Data DOP,
or connected directly to a data input of another data node, to
attach the data from this node to an object or another piece of
data.
Locals
channelname
This DOP node defines a local variable for each channel and parameter on the Data Options page, with the same name as the channel. So for example, the node may have channels for Position (positionx, positiony, positionz) and a parameter for an object name (objectname).
Then there will also be local variables with the names positionx, positiony, positionz, and objectname. These variables will evaluate to the previous value for that parameter.
This previous value is always stored as part of the data attached to the object being processed. This is essentially a shortcut for a dopfield expression like:
dopfield($DOPNET, $OBJID, dataName, "Options", 0, channelname)
If the data does not already exist, then a value of zero or an empty string will be returned.
DATACT
This value is the simulation time (see variable ST) at which the current data was created. This value may not be the same as the current simulation time if this node is modifying existing data, rather than creating new data.
DATACF
This value is the simulation frame (see variable SF) at which the current data was created. This value may not be the same as the current simulation frame if this node is modifying existing data, rather than creating new data.
RELNAME
This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP).
In this case, this value is set to the name of the relationship the data to which the data is being attached.
RELOBJIDS
This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP).
In this case, this value is set to a string that is a space separated list of the object identifiers for all the Affected Objects of the relationship to which the data is being attached.
RELOBJNAMES
This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP).
In this case, this value is set to a string that is a space separated list of the names of all the Affected Objects of the relationship to which the data is being attached.
RELAFFOBJIDS
This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP).
In this case, this value is set to a string that is a space separated list of the object identifiers for all the Affector Objects of the relationship to which the data is being attached.
RELAFFOBJNAMES
This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP).
In this case, this value is set to a string that is a space separated list of the names of all the Affector Objects of the relationship to which the data is being attached.
ST
This value is the simulation time for which the node is being evaluated.
This value may not be equal to the current Houdini time
represented by the variable T, depending on the settings of the 
DOP
Network Offset Time and Time Scale
parameters.
This value is guaranteed to have a value of zero at the
start of a simulation, so when testing for the first timestep of a
simulation, it is best to use a test like $ST == 0 rather than
$T == 0 or $FF == 1.
SF
This value is the simulation frame (or more accurately, the simulation time step number) for which the node is being evaluated.
This value may not be equal to the current Houdini frame number
represented by the variable F, depending on the settings of the DOP
Network parameters. Instead, this value is equal to
the simulation time (ST) divided by the simulation timestep size
(TIMESTEP).
TIMESTEP
This value is the size of a simulation timestep. This value is useful to scale values that are expressed in units per second, but are applied on each timestep.
SFPS
This value is the inverse of the TIMESTEP value. It is the number of timesteps per second of simulation time.
SNOBJ
This is the number of objects in the simulation. For nodes that
create objects such as the 
Empty Object node,
this value will increase for each object that is evaluated.
A good way to guarantee unique object names is to use an expression
like object_$SNOBJ.
NOBJ
This value is the number of objects that will be evaluated by the current node during this timestep. This value will often be different from SNOBJ, as many nodes do not process all the objects in a simulation.
This value may return 0 if the node does not
process each object sequentially (such as the 
Group
DOP).
OBJ
This value is the index of the specific object being processed by the node. This value will always run from zero to NOBJ-1 in a given timestep. This value does not identify the current object within the simulation like OBJID or OBJNAME, just the object’s position in the current order of processing.
This value is useful for generating a
random number for each object, or simply splitting the objects into
two or more groups to be processed in different ways. This value
will be -1 if the node does not process objects sequentially (such
as the Group DOP).
OBJID
This is the unique object identifier for the object being processed. Every object is assigned an integer value that is unique among all objects in the simulation for all time. Even if an object is deleted, its identifier is never reused.
The object identifier can always be used to uniquely identify a given object. This makes this variable very useful in situations where each object needs to be treated differently. It can be used to produce a unique random number for each object, for example.
This value is also the best way
to look up information on an object using the dopfield expression
function. This value will be -1 if the node does not process objects
sequentially (such as the Group DOP).
ALLOBJIDS
This string contains a space separated list of the unique object identifiers for every object being processed by the current node.
ALLOBJNAMES
This string contains a space separated list of the names of every object being processed by the current node.
OBJCT
This value is the simulation time (see variable ST) at which the current object was created.
Therefore, to check if an object was created
on the current timestep, the expression $ST == $OBJCT should
always be used. This value will be zero if the node does not process
objects sequentially (such as the Group DOP).
OBJCF
This value is the simulation frame (see variable SF) at which the current object was created.
This value is equivalent to using the
dopsttoframe expression on the OBJCT variable. This value will be
zero if the node does not process objects sequentially (such as the
Group DOP).
OBJNAME
This is a string value containing the name of the object being processed.
Object names are not guaranteed to be unique within a simulation. However, if you name your objects carefully so that they are unique, the object name can be a much easier way to identify an object than the unique object identifier, OBJID.
The object name can
also be used to treat a number of similar objects (with the same
name) as a virtual group. If there are 20 objects named "myobject",
specifying strcmp($OBJNAME, "myobject") == 0 in the activation field
of a DOP will cause that DOP to operate only on those 20 objects. This
value will be the empty string if the node does not process objects
sequentially (such as the Group DOP).
DOPNET
This is a string value containing the full path of the current DOP Network. This value is most useful in DOP subnet digital assets where you want to know the path to the DOP Network that contains the node.
Note
Most dynamics nodes have local variables with the same names as the
node’s parameters. For example, in a Position node,
you could write the expression:
$tx + 0.1
…to make the object move 0.1 units along the X axis at each timestep.
Examples
The following examples include this node.
PaintedGrog Example for Fluid Object dynamics node
RestartFluid Example for Fluid Object dynamics node
TimelessGas Example for Gas Particle to Field dynamics node
ColourAdvect Example for Fluid Source geometry node
| See also |
Vector Field Visualization
